Oncogene-induced sensitization to chemotherapy-induced death requires induction as well as deregulation of E2F1

The analysis of DNA tumor viruses has provided landmark insights into the molecular pathogenesis of cancer. A paradigm for this field has been the study of the adenoviral E1a protein, which has led to the identification of proteins such as p300, p400, and members of the retinoblastoma family. Through binding Rb family members, E1a causes deregulation of E2F proteins-an event common to most human cancers and a central pathway in which oncogenes, including E1a, sensitize cells to chemotherapy-induced programmed cell death. We report here, however, that E1a not only causes deregulation of E2F, but importantly that it also causes the posttranscriptional upregulation of E2F1 protein levels. This effect is distinct from the deregulation of E2F1, however, as mutants of E2F1 impaired in pRb binding are induced by E1a and E2F1 induction can also be observed in Rb-null cells. Analysis of E1a mutants selectively deficient in cellular protein binding revealed that induction of E2F1 is instead intrinsically linked to p400. Mutants unable to bind p400, despite being able to deregulate E2F1, do not increase E2F1 protein levels and they do not sensitize cells to apoptotic death. These mutants can, however, be complemented by either the knockdown of p400, resulting in the restoration of the ability to induce E2F1, or by the overexpression of E2F1, with both events reenabling sensitization to chemotherapy-induced death. Due to the frequent deregulation of E2F1 in human cancer, these studies reveal potentially important insights into E2F1-mediated chemotherapeutic responses that may aid the development of novel targeted therapies for malignant disease